1. Field of the Invention
The present invention relates to a method and an apparatus for measuring a refractive index distribution of an object such as an optical element.
2. Description of the Related Art
Optical elements such as lenses that are used for optical apparatuses such as digital cameras and laser beam printers require high refractive indices. On the other hand, molding technology enables easy production of complex shapes such as aspheric surfaces even when using optical glasses and plastics having high refractive indices.
However, molding sometimes causes, inside the optical element, refractive index nonuniformity depending on molding conditions. Such internal refractive index nonuniformity greatly affects optical characteristics of the optical element, which may make it impossible to obtain desired optical characteristics. Therefore, highly accurate measurement of internal optical homogeneity of the optical element having a high refractive index is required.
Methods of measuring the optical homogeneity generally include interferometries that measure a transmitted wavefront of an object (optical element) highly accurately processed to measure an internal refractive index distribution thereof. Moreover, a method is proposed which places an object between glass plates and soaks the object in oil being provided therebetween and having a refractive index approximately equal to that of the object to reduce surface accuracy errors of the object.
Japanese Patent Laid-Open No. 01-316627 discloses a method which measures a transmitted wavefront of an object soaked in a medium (matching oil) having a refractive index approximately equal to that of the object to obtain optical properties of the object. This method enables measurement of an internal refractive index distribution of the object without accurate processing of the object.
Japanese Patent Laid-Open No. 02-008726 discloses a method which measures a transmitted wavefront of an object soaked in a first matching oil having a refractive index approximately equal to that of the object, and further measures a transmitted wavefront of the object soaked in a second matching oil having a refractive index slightly different from that of the object. The disclosed method obtains a shape and a refractive index distribution of the object based on the transmitted wavefronts measured with the first and second matching oils.
In the measurement with the second matching oil, influences of the refractive index distribution and shape of the object appear as interference fringes on a detector for measuring the transmitted wavefront. Therefore, it is necessary that the refractive index of the second matching oil be slightly different from that of the object within a range where the interference fringes are not exceedingly thickened.
The measuring methods disclosed in Japanese Patent Laid-Open Nos. 01-316627 and 02-008726 need the matching oil having the refractive index approximately equal to that of the object. However, matching oils having high refractive indices generally have low transmittances. Therefore, when measuring a transmitted wavefront of an object whose refractive index is high by the measuring methods disclosed in Japanese Patent Laid-Open Nos. 01-316627 and 02-008726, the detector outputs only a small signal, which deteriorates measurement accuracy.
On the other hand, when using a low refractive index matching oil, aberrations caused due to the shape of the object are added to the transmitted wavefront. Moreover, since light being used for the measurement becomes non-collimated light, arrangement errors of optical elements other than the object also affect the transmitted wavefront. These make it difficult to accurately extract only the refractive index distribution from the transmitted wavefront.